The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 1.7286\\[1 em]x_2 &= -3.4389\\[1 em]x_3 &= 4.7102 \end{aligned} $$Polynomial $ x^3-3x^2-14x+28 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.